1. Field of the Invention
The present invention relates to a method and apparatus for improving the performance of a modulator crystal in an imaging assembly. In particular, the invention relates to a method and apparatus for heating a modulator crystal in an imaging assembly without substantially modifying the temperature of the electronic components of the imaging assembly.
2. Background Information
Some of the current trends in the thermal offset printing plate industry have been in the area of increased productivity, especially as they relate to so-called xe2x80x9cComputer to Platexe2x80x9d (CTP) systems. However, such conventional systems are presently limited, especially as they relate to imaging of thermal offset plates. Conventional internal drum systems are limited, for example, with respect to the spinning speed of the mirror, the commutation time on/off of the laser beam (for acousto-optic modulators with YAG lasers, red and UV laser diodes and optical fiber lasers), and power of the laser sources. Conventional external drum systems which have a plurality of laser sources such as diodes are limited, for example, with respect to respective rotational speeds, respective number of diodes and the total power generated thereby. Conventional external drums employing a spatial modulator also have power limitations as well as limitations with respect to the number of spots produced thereby. Conventional flat bed systems have xe2x80x9cwidth of platexe2x80x9d limitations, resolution limitations, as well as limited scanning speeds, modulation frequencies and power of the respective laser source.
A conventional system in which a laser beam is widened in one dimension to cover an array of a substantial number of electro-optic gates (so that a large number of adjacent spots can be formed and thus constitute a xe2x80x9cwide brushxe2x80x9d) is described in U.S. Pat. No. 4,746,942, which is incorporated herein by reference. In particular, this patent discloses that the beam is divided by the gates into a plurality of potential spot-forming beams. The transmission of each beam to a photosensitive surface for imaging is selectively inhibited in accordance with a pre-determined pattern or program, while the beams are swept relative to the photosensitive surface to form characters and other images.
However, the number of spots of the brush described in this patent may be limited by optical aberrations. In addition, the power that a single laser source can produce limits the imaging speed of thermo-sensitive plates because of their low sensitivity. The performance of a spatial modulator with a single laser source can also be limited.
Laser modulators dedicated to the exposure of thermal media require a laser power of tens of watts or more. In contrast, Total Internal Reflection (TIR) modulators are especially well suited for the exposure of heat sensitive plates, as described, for example, in Electronic Letters, Vol. 9 (1973), p. 309. TIR modulators based on the use of LiNbO3 crystals are of particular interest because of their commutation speed. This type of modulator is described in the literature and several patents such as in U.S. Pat. No. 4,281,904, which is incorporated herein by reference. The operation of TIR modulators in laser printers has been described in U.S. Pat. No. 4,554,561, which is incorporated herein by reference. TIR modulators are presently used in platesetters for the production of heat-sensitive printing plates where a high level of energy is necessary, and the implementation of TIR modulators in platesetters has been described in U.S. patent application Ser. No. 09/865,345 [KPG 1117], filed May 25, 2001, which is incorporated herein by reference.
Since a TIR modulator does not absorb energy, rays of high-power laser may penetrate the modulator without producing heat. However, the penetration of electro-optic modulators by high-energy rays has a negative effect, as described in SPIE Electro-Optic and Magneto-Optic Materials, Vol. 1274 (1990), especially in TIR modulators such as those described in U.S. Pat. No. 4,281,904. When the crystal is submitted to high-energy radiation, the strong energy density induces detrimental photorefraction effects known as xe2x80x9coptical damagexe2x80x9d and xe2x80x9cdc drift.xe2x80x9d The presence of these side effects in systems where a high level of energy is necessary limits the amount of energy to which a modulator can be subjected, such as exposure of so-called thermal plates.
The present invention addresses the presence of optical damage and DC drift in a modulator. Both problems are solved by raising the operating temperature of the modulator and maintaining the temperature within a predetermined range. The invention also provides a method and apparatus associated for stabilizing the performance of a modulator crystal by automatically controlling the operating temperature of the crystal. The invention also provides a method and apparatus for protecting electronic components associated with a modulator crystal from the detrimental effects of the high temperature of the crystal. Other objects, features and advantages of the present invention will be apparent from the detailed description provided herein.
Accordingly, it is an object of the present invention to provide an imaging assembly comprising: (a) a modulator crystal comprising a first surface and a second surface substantially opposite to the first surface, wherein the first surface comprises an active area; and (b) a heating element for heating the modulator crystal to a temperature within a predetermined temperature range, wherein the heating element is positioned under the modulator crystal and comprises a first surface, wherein the heating element first surface faces the modulator crystal second surface and covers a portion of the modulator crystal second surface such that the active area of the first surface of the modulator crystal has a homogeneous temperature.
It is another object of the present invention to provide a method for heating a modulator crystal in an imaging assembly, the method comprising: (a) providing a modulator crystal comprising a first surface and a second surface substantially opposite to the first surface, wherein the first surface comprises an active area; (b) providing a heating element comprising a first surface, wherein the heating element first surface faces the modulator crystal second surface and covers a portion of the modulator crystal second surface such that the active area of the first surface of the modulator crystal has a homogeneous temperature; (c) heating the modulator crystal with the heating element to a temperature within a predetermined temperature range; and (d) maintaining the temperature of the modulator crystal within the predetermined temperature range.
It is another object of the present invention to provide an imaging assembly comprising: (a) a modulator crystal comprising a first surface and a second surface substantially opposite to the first surface, wherein the first surface comprises an active area; (b) a heating element for heating the modulator crystal to a temperature within a predetermined temperature range, wherein the heating element is positioned under the modulator crystal and comprises a surface, wherein the heating element surface faces the modulator crystal second surface and covers a portion of the modulator crystal second surface such that the active area of the first surface of the modulator crystal has a homogeneous temperature; and (c) one or more electronic components residing above the first surface of the modulator crystal, wherein the electronic components are thermally insulated from the modulator crystal and from the heating element.
It is another object of the present invention to provide a method for heating a modulator crystal in an imaging assembly, the method comprising: (a) providing a modulator crystal comprising a first surface and a second surface substantially opposite to the first surface, wherein the first surface comprises an active area; (b) positioning under the modulator crystal a heating element comprising a first surface, wherein the heating element first surface faces the modulator crystal second surface and covers a portion of the modulator crystal second surface such that the active area of the first surface of the modulator crystal has a homogeneous temperature; (c) providing one or more electronic components residing above the first surface of the modulator crystal, wherein the electronic components are thermally insulated from the modulator crystal and from the heating element; (d) heating the modulator crystal with the heating element to a temperature within a predetermined temperature range; and (e) maintaining the temperature of the modulator crystal within the predetermined temperature range.
It is another object of the present invention to provide an imaging assembly comprising: (a) a modulator crystal comprising a first surface and a second surface substantially opposite to the first surface, wherein the first surface comprises an active area; (b) a heating element for heating the modulator crystal to a temperature within a predetermined temperature range, wherein the heating element contacts and covers a portion of the modulator crystal second surface such that the active area of the first surface of the modulator crystal has a homogeneous temperature; and (c) one or more electronic components positioned under the first surface of the modulator crystal, wherein the electronic components are thermally insulated from the modulator crystal and from the heating element.
It is another object of the present invention to provide a method for heating a modulator crystal in an imaging assembly, the method comprising: (a) providing a modulator crystal comprising a first surface and a second surface substantially opposite to the first surface, wherein the first surface comprises an active area; (b) contacting and covering with a heating element a portion of the modulator crystal second surface such that the active area of the first surface has a homogeneous temperature; (c) providing one or more electronic components positioned under the first surface of the modulator crystal, wherein the electronic components are thermally insulated from the modulator crystal and from the heating element; (d) heating the modulator crystal with the heating element to a temperature within a predetermined temperature range; and (e) maintaining the temperature of the modulator crystal within the predetermined temperature range.
The present invention ensures that the crystal of the TIR modulator is heated to an optimal temperature while at the same time other components of the imaging assembly are thermally insulated. These components include electronic components associated with the modulator crystal. The temperature of these electronic components is maintained within the temperature range defined by operating specifications of the electronic components. The temperature within the modulator crystal is maintained homogeneous, thereby avoiding undesirable variations of the index of refraction within the crystal due to temperature variations within the crystal.